The present invention relates to a pattern forming method.
In recent years, a chemical amplification resist containing an acid generator is widely used for fine processing of a semiconductor device. FIG. 1 shows that an underlying film 2 and a chemical amplification positive resist layer 3 are formed on a substrate 1. As shown in the drawing, an acid (H.sup.+) is generated upon light exposure from the acid generator contained in the resist layer 3 so as to cause the solubilization of the resin, which is a main component of the resist, to be chemically amplified. On the other hand, when it comes to a chemical amplification negative resist, the acid generated upon light exposure permits the insolubilization of the resin to be chemically amplified. As a result, the sensitivity to exposure to light is improved.
However, it is known to the art that footing and undercut are observed in the pattern formed by selectively exposing a chemical amplification resist to light, followed by developing the light-exposed pattern.
In the case of a chemical amplification positive resist, the footing is caused by the diffusion of the base species present on the surface of the underlying film into the resist layer. To be more specific, when an underlying film is formed on a substrate, followed by forming a chemical amplification positive resist layer on the underlying film and subsequently exposing selectively the resist to light, an acid is generated from the acid generator contained in the resist. In this case, if the base species present on the surface of the underlying film 2 is diffused into the resist layer 3, the acid generated in the vicinity of the interface between the resist layer 3 and the underlying film 2 is neutralized. As a result, the resin forming the resist layer 3 is prevented from solubilizing in the vicinity of the interface between the resist layer 3 and the underlying film 2. It follows that the resist is left unremoved at the edge portion of the pattern obtained in the subsequent developing step, giving rise to the footing problem as shown in FIG. 2. The footing is also brought about in the case where the acid generated in the resist layer is diffused into the underlying film.
On the other hand, the undercut is generated by diffusion of the acid species present on the surface of the underlying film into the resist layer. To be more specific, if the acid present on the surface of the underlying film 2 is diffused into the resist layer 3 in the light-exposure step, an additional acid other than the acid generated from the acid generator contained in the resist layer 3 is present within the resist layer so as to cause the resin forming the resist layer 3 to be made soluble. As a result, the resist is removed excessively at the edge portion of the pattern in the subsequent developing step so as to give rise to the undercut problem, as shown in FIG. 3.
In the case of a chemical amplification negative resist, the mechanisms of the undercut and footing problems are opposite to those described above in conjunction with the chemical amplification positive resist. Specifically, the undercut and footing problems are brought about in the cases where the base and acid species are diffused from the underlying film into the resist layer, respectively.
It should also be noted that, where, for example, a chemical amplification positive resist layer is selectively exposed to light and, then, developed, a so-called T-topping, i.e., the problem that a basic contaminant within the atmosphere causes the resist to be left unremoved in an upper portion of the resist pattern, is observed in some cases.
If the footing or undercut takes place in the resist layer, it is impossible to pattern the underlying film in a predetermined size, making it difficult to apply a fine processing to a semiconductor device.
A measure for overcoming the above-noted difficulty is proposed in, for example, U.S. Pat. No. 5,372,914. It is proposed that a protective layer consisting of, for example, a thermosetting resin should be formed between a substrate and a resist layer in order to suppress the diffusion of a base species from the substrate. For suppressing the diffusion of a base species, however, it is necessary to coat the thermosetting resin in a sufficient thickness by means of spin coating, CVD, etc. Also, for removing the thermosetting resin from the space region, it is necessary in some cases to use an etchant differing from the developing solution of the resist. It follows that the pattern forming process is rendered complex in the case of forming a protective layer as proposed in the U.S. Patent quoted above.